A fuel cell is an electrochemical device that converts chemical energy of a fuel (such as hydrogen, methanol, ethanol) and oxidant (oxygen from air) into electrical energy and heat. The fuel cell has all the attributes of a battery, except that a fuel cell continues to produce electricity as long as fuel and oxidant are available, as opposed to a battery that stops producing power when the stored chemicals are exhausted. Several different types of fuel cells are under development. Amongst these, polymer electrolyte membrane (PEM) fuel cell is regarded as the most suitable technology for transport and small scale distributed power generation applications, because they operate at low temperatures (70-80° C.) and offer rapid start and shut down, unlimited thermal cycling capability and excellent load following characteristics. Around 50% of the power is available at cold start. A conventional polymer electrolyte membrane fuel cell stack consists of a number of cells called membrane electrode assemblies (MEAs) assembled together in series with the help of interconnect (bipolar middle and unipolar end ones) plates to produce the required stack voltage and power. Each cell (or MEA) consists of a proton conducting polymer membrane sandwiched between a hydrogen (anode) electrode and an oxygen (cathode) electrode. The interconnect plates serve dual purpose: to electrically connect one cell to the other (to conduct electrical current) and to distribute reactants (as well collect products) to (from) the respective electrodes of the MEAs. Hydrogen and air (source of oxygen) are supplied to the electrodes via flow field gas channels in the interconnect plates. On shorting the cell (or stack) through an external load hydrogen supplied to the anode gets oxidised to protons and electrons. Electrons travel through the external load and protons are transported through the membrane to the cathode, where they react with the oxygen supplied to cathode side and electrons from the external load to produce water as per following reactions.At anode (Hydrogen electrode): H2=2H++2e At cathode (Air electrode): 2H++½O2+2e=H2O
The oxygen-depleted air along with the water formed on the air side of the MEA electrodes are collected by the gas flow channels. The air supplied to the oxygen electrode, in addition to supplying oxygen, also helps in the removal of water formed at the electrode and thereby uncovering the reaction sites for more oxygen (air) access for the reaction. The voltage from a single cell under load conditions is in the range of 0.5 to 1.0V DC and current densities in the range 100 to 700 mA.cm−2.
In the case of micro fuel cells for portable power applications, the fuel cell system is required to be smaller, simpler (without or less moving parts) and easily manufacturable at mass scale. This is where the concept of self air breathing (no air compressors for oxygen supply to fuel cell), passive operation (no moving parts), miniaturisation of components (interconnects, micro fluid flow channels, overall system) and cheap fabrication methods have to be introduced to compete with batteries.
The material used for interconnect is required to have relatively high electrical conductivity, high corrosion resistance, low contact resistance and impervious to supplied fuel and oxidant. The conventional method of fabricating interconnects for fuel cells is by machining the flow channels into any electrically conducting material such as graphite or steel etc. The machining process is labour intensive and there is a limitation on the geometry and minimum size of the channels that can be made. Moreover, mass production techniques are required to cater for low cost and large volume market (e.g. battery replacement).
Examples of interconnect plate manufacture are known. For example, U.S. Pat. Nos. 7,147,677, 6,858,341, 6,051,331 and 5,858,567 provide examples of interconnect plates.